To ensure a reliable operation under the above-mentioned conditions, composite self-lubricating materials should meet the following requirements:
they should have a high mechanical strength at high and low temperatures, under thermal shocks;
they should be sufficiently corrosion-resistant under the conditions of high humidity and elevated temperature, radiation-resistant;
they should be stable in operation: have a low friction coefficient, low wear intensity, long life and reliable operation;
they have to be processable in the manufacture, assembling, servicing.
Furthermore, for such materials to be operated under the conditions of electrical fields an important factor is the stability of electrical characteristics of these materials, e.g. level of pulsation thereof, contact resistance, etc.
Known in the art are various mating parts of friction units of composite self-lubricating materials manufactured by methods of powder metallurgy: bearings containing metals such as copper, iron, brass and functional additives (cf. USSR Inventor's Certificates Nos. 144015 B-1, 1962, 158824 B-22, 1963, 160298 B-3, 1964; 419318 Cl. B 22 f 3/12; 568076 Cl. B 22 f 3/12, C 22 c 1/04; British patent application No. 1,483,328 Cl. B 22 f 7/04, F 16 33/06; U.S. Pat. Nos. 4,004,889 Cl. B 22 f 7/04, 4,049,428 Cl. B 22 f 1/02; Japanese Patent Application Nos. 52-15241, Cl. B 22 f 5/26, 52-2604 Cl. B 22 f 3/34, 52-26737 Cl. B 22 c 9/02 and others).
Also known are electrical brushes for electric motors manufactured from composite self-lubricating materials containing metals such as silver, copper and functional additives (cf. USSR Inventor's Certificates Nos. 312314 Cl. C 22 c 5/00, 599228 Cl. B 22 f; U.S. Pat. Nos. 2,418,710, 1944; 2,854,597, 1955; 3,437,592 Cl. 252-12, No. 3,455,023 Cl. HOlr 4,056,365 Cl. B 22 f 3/00; French Patent Nos. 1,203,011; Cl. H 02 k, 1,306,535 Cl. H 02 k; 1,392,967 Cl. H 02 f; FRG Patent Nos. 1,215,938 Cl. 40 B 5/00, 2,712,209 Cl. B 22 f 5/00, No. 2,715,347, Cl. B 22 f 5/00) and other articles.
As the functional additives use is made of:
1. polymers:
thermostable ones: nitrogen--or phosphorous-containing (U.S. Patent No. 4,045,400 Cl. C 08 k 3/30, 9/02), epoxy (British Patent Appl. No. 1,479,402 Cl. C 08 G 59/11) and others;
organic binders (cf. French Appl. Nos. 2,377,750 and 2,337,751 Cl. C 08 L 53/00; B 22 f 3/00, C 04 B 35/64);
2. fibres:
graphite, synthetic materials, metals (both oriented and not with a size of from 1 to 100 .mu.nm in an amount of from 20 to 80% by volume), e.g. FRG Patent No. 1,783,124 Cl. 4 OB 1/10, C 22 c 1/10;
3. oxides of metals, e.g. cadmium oxide (British patent application No. 1,486,011 Cl. C 08 L 27/18);
4. metals (Fe, Ni, Ag, Pb, Cd, In, Al), metal carbides (WC, FeC), oxide components (1-20%) e.g. osmium tetraoxide contained mainly in a solid solution with the basic component (cf. U.S. Pat. No. 3,370,942 Cl. C 22 c; FRG Pat. No. 1,249,536 Cl. 40 c 5/00;
5. lubricants:
antifriction corrosion-resistant solid lubricants based on a thermosetting resin, as well as graphite, molybdenum disulphide, zinc chromate or metals: cadmium, silver, tungsten (U.S. Pat. No. 3,051,586 Cl. 117-49);
high-temperature lubricants containing lubricating agents (molybdenum disulphide, boron nitride or graphite), metals (copper, zinc, indium or lead) and cadmium oxide with a particle size of not more than 10 .mu.m (cf. French Pat. No. 1,345,589 Cl. C 01 m);
based on polymers such as products of condensation of an aromatic epoxy resin in an amount of from 15 to 60% by weight, solid lubricants in an amount of from 40 to 85% by weight, graphite, oxides of metals (cf. FRG Pat. No. 1,271,875 Cl. c 1/01 23, C 01 m);
diselenides of transition metals (tantalum diselenide, tantalum sulphide, niobium diselenide and mixtures thereof) for operation in a high vacuum of the order of below 10.sup.-7 torr (cf. FRG Pat. No. 1,284,017 Cl. C 10 m).
Composite self-lubricating materials, for example electrically conducting ones, are produced mainly by methods of power metallurgy, in particular by the solid-phase sintering method.
Known in the art is a composite self-lubricating material (cf. USSR Inventor's Certificate No. 347845, Cl. H Ol r 39/20) employed for the manufacture of electric brushes operating mainly in a high vacuum at negative temperatures. To improve operation characteristics, into the material containing metallized carbon powder, particles of an antifriction agent and a binder, metallized molybdenum disulphide is additionally introduced in an amount of from 40 to 70% by weight; molybdenum disulphide per se contains 30 to 50% by weight of the metal. As an example the following composition is described: 50% by weight of natural copper-coated graphite, 50% of copper-clad molybdenum disulphide containing 30% by weight of copper, as well as resol-type phenol-formaldehyde resin in the amount of 20% by weight.
Electric brushes manufactured from this material by hot compression-moulding have the following characteristics: volume resistivity 2.5-2.8 Ohm.mm.sup.2 /m. hardness 33-43 kgf/mm.sup.2, transition voltage drop per pair of brushes (copper ring)--1.3-1.4 V. Wear in vacuum of 4.10.sup.-6 torr at the temperature of -75.degree. C. for 30 hours is 0.02 mm.
From the above-given data it follows that, despite the whole range of merits, namely high wear-resistance in vacuum, erosion resistance, a low friction coefficient, the above-described material possesses properties which do not enable its full use under extreme operation conditions:
presence of a filler, though ensuring lowered friction coefficient, impairs other properties such as vibration and impact-resistance;
the material has a low mechanical strength, e.g. upon compression due to the fact that a greater portion of a mechanical load borne by the material is distributed over the interface metal-filler. The adhesion strength in the contact metal-filler is rather low, which is due to the procedure of manufacture of the material;
a high transition voltage drop in the contact: electric brush-collector is responsible for electric losses, noises, high current fluctuations;
impaired operation performances of the material after a long storage period (10 days and longer) in the static state and under high humidity conditions (98%) and elevated temperature of 40.degree. C. In this case there is observed "sticking" of the material to the metallic mating body, e.g. copper one. At the same time, electric resistance in the contact electric brush-collector is sharply increased and the voltage of the electric machine starting is inadmissibly increased by as much as 3 to 5 times.
Due to the above-mentioned reasons the material has a low operation reliability.
Known in the art is a number of composite self-lubricating materials based on a metal matrix, which possess higher service characteristics.
A material of such type is known, which consists of a porous bronze matrix formed by sintering of metal powders so that in pores of the metal matrix functional additives are present, for example polytetrafluorethylene (cf. F. P. Bowden, Frictional Properties of Porous Metals Impregnated with Plastics--Research, 1950, v.3, p.147).
There is also known another composite self-lubricating material for strip bearings (British Pat. No. 1,114,061; B 912121, 756950).
The material comprises a porous metal matrix with a pore volume of up to 35%, randomly filled with functional additives. As the matrix use is made of metals such as high tin-content bronze, while as the functional additives polytetrafluorethylene and lead are used. The material is produced by methods of powder metallurgy, for example by sintering, from the electrolytically produced dendrite powders of pure metals of spherical particles of powder of high tin-content bronze containing up to 10% by weight of tin. The process for the manufacture of such materials involves: high-temperature compression of the powder, its preliminary sintering in a reducing atmosphere to a semi-finished product, repeated impregnation of this semi-finished article with a suspension of functional additives, final sintering of the impregnated semi-finished article, e.g. together with the steel base, in an inert medium and calibration of the resulting article by plastic deformation (upsetting). In the case of manufacturing articles from this materials, e.g. strip bearings, it is necessary that at the stage of impregnation of the matrix with functional additives the latter be extending above the article surface as a layer with a thickness of about 0.02 mm, which is an indispensable condition for a high operability of the bearing.
The materials are produced as strips with a thickness of 1.21 to 3.05 mm and width of up to 101.6 mm; bushings with a diameter of 10 to 100 mm, semi-spherical supports with a diameter of from 16 to 38 mm, wire, bearings, etc. The material is employed under the conditions of dry and semi-liquid friction.
These materials possess a high operability (low wear intensity and low friction coefficient) including that under heavy loads, without lubrication within a wide temperature range (from -200 to +260.degree. C.) in the air and other gas media, in vacuum, as well as in liquid media possessing no lubricating effect. No static charges are formed on these materials; neither they cause fretting corrosion.
One of the advantages of these materials resides in the feasibility of using them in friction units operating under extreme conditions.
It is known that these materials are used as cage members of ball bearings made of sintered silver impregnated with polytetrafluorethylene with addition of a small amount of tungsten diselenide which successfully operate at low and high temperatures in a high vacuum of from 10.sup.-7 to 10.sup.-9 torr. The materials find extensive use in the following industries: aviation and space engineering, automobile industry, electrical engineering, textile and other industries.
The material, however, has a limited range of applications due to the fact that
(a) not all pores in the material are uniformly impregnated, since they differ in size, position and, hence, during the operation of the material, lubrication stock in the pores adjacent, at the moment, to the friction surface, is exhausted;
(b) pores may become clogged due to contamination thereof with the products of wear, foreign matter, as well as due to plastic deformation of superficial layers of the matrix, e.g. at high loads and temperatures;
(c) service characteristics of the material depend on its physico-mechanical, frictional and other characteristics within the contact microareas. The same characteristic may have different values within different areas and vary within a wide range;
(d) adhesion interaction between the functional additive (polytetrafluoroethylene is chemically inert to metals) and the metal matrix during impregnation is rather insignificant. This lowers mechanical strength characteristics of the material and, eventually, impairs numerous final characteristics such as wear-resistance in vacuum, vibration-and impact-resistance, electrical conductivity and the like, i.e. reliability;
(e) the material has a low thermal stability (up to +280.degree. C.) due to destruction of polytetrafluoroethylene;
(f) the material has a low resistance to irradiation in air due to destruction of the binder;
(g) the procedure for the manufacture of the material requires a whole range of auxiliary operations: intimate intermixing, repeated impregnation, multiple compression-moulding and sintering of metal powders, a plurality of finishing operations: calibration, complicated stepwise thermal treatment, polishing, and the like.